Linear variable displacement transformers (LVDTs) are frequently employed to monitor differential motion in apparatus. A typical prior art LVDT includes an E-shaped core having a primary coil disposed around its center leg and two secondary coils, each disposed around one of the other two legs, at opposite ends of the core. A pole piece, which is approximately as long as the core, carries magnetic flux between the center leg and both of the end legs. Very small gaps separate the pole piece from the outwardly facing ends of each of the three legs of the core. As the pole piece moves from one end of the core toward the other end, it couples more of the magnetic flux through one of the end legs than through the other, creating a differential electrical signal (voltage or current) in the two secondary coils. A circuit connected to the two secondary coils demodulates the differential electrical voltage or current, producing a signal indicative of the displacement of the two parts of the apparatus, i.e., the displacement of the pole piece relative to the core.
One of the problems associated with a conventional LVDT concerns its sensitivity to rotation of the pole piece with respect to the core. As the pole piece rotates, the gap between one of the end legs and the pole piece decreases and the gap between the other end leg and pole piece increases. Accurate detection of the relative linear displacement of the pole piece and the core is predicated on the assumption that the separation or gap between the pole piece and each end leg does not vary. The reluctance between the pole piece and each end leg is proportional to this gap distance divided by the cross-sectional area of the end leg overlapped by the pole piece. The relative displacement of the pole piece with respect to the core should therefore change only the cross-sectional area of the overlap, not the gap distance; however, motion of the apparatus to which the pole piece is attached may inherently cause the pole piece to rotate slightly. Rotation of the pole piece distorts the differential signal from the secondary coils so that it no longer accurately indicates the longitudinal displacement of the pole piece relative to the core.